This invention pertains generally to the control of core operation of nuclear reactor and more particularly, to the control of the axial power distribution and core power level of a nuclear reactor having a negative reactivity moderator temperature coefficient.
Generally, nuclear reactors contain a reactive region commonly referred to as the core in which sustained fission reactions occur to generate heat. The core includes a plurality of elongated fuel rods comprising fissile material, positioned in assemblies and arranged in a prescribed geometry governed by the physics of the nuclear reaction. Neutrons bombarding the fissile material promote the fissionable reaction which in turn releases additional neutrons to maintain a sustained process. The heat generated in the core is carried away by a cooling medium, which circulates among the fuel assemblies and is conveyed to heat exchangers which in turn produce steam for the production of electricity.
Commonly in pressurized water reactors a neutron absorbing element is included within the cooling medium (which also functions as a moderator) in controlled variable concentrations to modify the reactivity and thus the heat generated within the core, when required. In addition, control rods are dispersed among the fuel assemblies, longitudinally movable axially within the core, to control the core's reactivity and thus its power output. Generally, in the past in pressurized water reactors there have been three types of control rods that have been employed for various purposes. Full-length rods, which extend in length to at least the axial height of the core, are normally employed for reactivity control. Part-length control rods, which have an axial length substantially less than the height of the core, are normally used for axial power distribution control. In addition, reactor shutdown control rods are provided for ceasing the sustained fission reaction within the core and shutting down the reactor. The part-length rods and full-length rods are arranged to be incrementally movable into and out of the core to obtain the degree of control desired.
As a by-product of the fission reaction, through a process of .beta. decay of radioactive iodine, Xenon is created. Xenon has the property of having a large neutron absorption cross-section and therefore has a significant effect on the power distribution within the core and reactivity control. While the other forms of reactivity management are directly responsive to control, the Xenon concentration within the core creates serious problems in reactor control in that it exhibits a relatively long decay period and requires up to at least 20 hours after a power change to reach a steady state value.
While the radial power distribution of the core is fairly uniform, due the prescribed arrangement of fuel assemblies and the positioning of control rods which are symmetrically situated radially throughout the core, the axial power distribution can vary greatly during reactor operation. The axial power distribution of the core can create many problems throughout the course of reactor operations. Normally coolant flow through the fuel assemblies is directed from a lower portion of the core to the upper core regions, resulting in a temperature gradient axially along the core. Changes in the rate of the fission reaction, which is temperature dependent, will thus vary along the axis of the core. Secondly, the axial variation in the power distribution varies the Xenon axial distribution, which further accentuates the variations in the power axially along the core. Thirdly, insertion of the full length control rods from the top of the core, without proper consideration of the past operating history of the reactor can add to the axial power asymmetry.
The change in reactor core power output which is required to accommodate a change in electrical output of an electrical generating plant is commonly referred to as load follow. One load follow control program currently recommended by reactor vendors utilizes the movement of the full-length control rods for power level increases and decreases and the part-length control rods to control axial oscillations and shape the axial power profile. Changes in reactivity associated with changes in the Xenon concentration are generally compensated for by corresponding changes in the concentration of the neutron absorbing element in the core coolant or moderator. In this mode of operation, the part-length rods are moved to maintain the axial offset within some required band, typically plus or minus fifteen percent. The axial offset is a useful parameter for measuring the axial power distribution and is defined as: EQU A.O.=(P.sub.t -P.sub.b)/P.sub.t +P.sub.b)
where P.sub.t and P.sub.b denote the fraction of power generated in the top half and the bottom half of the core respectively. Under such a load follow program, no effort is made to maintain the inherent core axial power profile. The part-length rods are moved to minimize and reduce the axial offset independent of the previously established steady state axial offset. This process induces a constant fluctuation of the axial offset during sustained load follow operations which results in a number of undesirable operating conditions. For one thing, power pinching, which is a large axially centered power peak, is likely to occur. Such power peaks result in a reactor power penalty which requires the reactor to be operated at a reduced power level so that such peaks do not exceed specified magnitudes. Secondly, severe changes occur in the axial power profile of a transient nature during large load changes due to heavy insertion of control rods at reduced power levels. Thirdly, large Xenon transients occur upon coming back to power resulting in occurrences such as axial power oscillations. Fourthly, the part-length rod broad operating instructions supplied by reactor manufacturers are generally vague and require anticipation and interpretation by the reactor plant operator. Fifthly, increased hot channel factors result (which are hot spots which occur within the cooling channels among the fuel assemblies) and require a reduction in the power rating of the reactor to accommodate severe transients and/or adverse power profiles. Under such load follow programs no protection exists against severe pinching with small axial offsets.
A new method of operation for a nuclear reactor, described in application Ser. No. 501,569, filed Aug. 29, 1974, has been proposed to avoid the aforegoing adverse operating characteristics. The proposed method maintains a substantially symmetric Xenon axial profile during normal reactor operation including load follow. Normal operation generally excludes startup of the reactor and reactor shutdown and is normally interpreted to include the power operating range of the reactor in response to load requirements. Implementation of the desired Xenon distribution in accordance with this method is obtained by monitoring the power generated in the core at a first and second axial location. The core power parameters measured at the two locations are computed in accordance with a predetermined relationship such as the axial offset to give a value indicative of the axial power distribution of the core. The reactivity control mechanisms of the reactor are manipulated in accordance with the monitored values to maintain a substantially symmetric power distribution within the core throughout reactor operation under power including load follow.
Two separate embodiments have been taught for maintaining the desired axial power distribution prescribed by this new method of reactor operation. In the first, the part-length control rods remain withdrawn from the core while the neutron absorbing element within the core coolant is employed to assist adjustment of the reactivity of the core to correspond to changes in output power requirements and the full-length control rods are manipulated to maintain the desired axial power profile. In the second embodiment, the full-length control rods are used to control the reactivity changes associated with changes in power in the core and the part-length rods are employed to control the axial power distribution, while the neutron absorbing element within the core coolant compensates for reactivity changes due to Xenon buildup or depletion. Each embodiment is capable of implementing the concepts of the method and each has its respective distinct advantages. For example, the operation with part-length control rods has certain advantages over operation with-out part-length control rods such as the ability to provide quick changes in output power and the easiness of axial offset control. One disadvantage of part-length rod operation is the burn-up shadowing that results when the part-length rods are positioned near the middle of the core during full power operation. Because part-length rods act as neutron absorbers, the fuel screened by the part-length rods depletes at a much lesser rate than the remaining core. This could result in high peaking near the center of the core when the part-length rods are withdrawn, if extended load operation is performed. Burn-up shadowing has become the subject of a growing concern and the use of part-length rods are being discouraged. Alternatively, operation without part-length rods, while desirable from a fuel efficiency standpoint, exhibits a relatively slow return to power capability in response to an increase in load. The rate of a power increase during operation without part-length rods is dependent upon the dilution rate of the neutron absorbing element within the coolant, which in the case of pressurized water reactors is boron. Presently, operating nuclear reactors employ ion exchange or coolant replacement systems to control the boron concentrations within the coolant. These systems typically have a relatively slow response time.
Accordingly, an improved method of operation of a nuclear reactor is desired that will increase the load follow capability of nuclear power plants to accommodate rapid excursions in load requirements.